Tuesday, April 21, 2009

Decisions, decisions, decisions

I've not really talked much about anything EV specific so far, mainly because I've been concentrating on figuring out how to tear the car apart - I figured that if that didn't go well there was no point in making any decisions about EV components. But with nearly all the ICE components out, the car not looking any worse then you'd expect for one with half it's parts missing and all my body parts still attached it's about time to make some decisions about what direction I'm going in - if I don't I'm sure my better half is eventually going to get a little bored of a hunk of useless metal taking up a garage bay.

Since starting my investigation I've been kicking around 2 possible configurations. The first is a high-voltage AC (alternating current) system consisting of:

- a 320V battery
- an Azure Dynamics AC24LS AC Motor
- the matching Azure Dynamics DMOC455 Controller

or, a DC (direct current) system consisting of

- a 144V battery
- a Netgain WarP9 DC Motor
- a Curtis 1231C-8601 Controller

I liked the high voltage AC option a lot as higher voltage means lower current for the same amount of power, and lower current means better efficiency and hence more range. It also includes 'regenerative braking' (using the motor as a generator to assist when braking, so putting some of the car's kinetic energy back into the battery instead of wasting it as heat from the brakes). But the Azure Dynamics system seems a little anemic when it comes to acceleration from feedback I've found in build logs on the internet.

DC motors are less efficient, and you need to go through a lot of hoops and have a special motor controller to get regenerative braking, but they are far more popular for DIY conversions. Popular opinion is that a DC system is simpler than AC (though as far as I can see, AC systems have more components built in so appear simpler to me!) and there are certainly a lot more examples out there to draw on when I get in trouble/need help so I'm leaning that way.

I was pushed to make a decision this past weekend when I was able to hook up with a group buy for batteries. One think I made my mind up about long ago was what battery technology I wanted to use. Most conversions use good old lead acid batteries - 12-20 or more in series - to create a battery pack of around 144V. But if you've ever changed a battery in your car you'll realize that this is one heck of a lot of weight to add. Even accounting for the 250-300lbs taken out in ICE components, you're talking adding 600-1000lbs to the car - that's going to be a real drag on both acceleration and range.

No - I decided that I would be going with some form of Lithium Ion-based battery. The problem with these batteries is quite simple - cost! There are various battery chemistries and a battery pack can cost between 4 and 10 (or more) times the equivalent nominal capacity in Lead Acid batteries. They also require a sophisticated Battery Management System (BMS) to ensure that the individual cells are not over-charged or over-discharged and remain in proper balance, adding more to the cost.

However they have some huge advantages over Lead Acid batteries. The amount of energy they can store is 2-3 times greater than the equivalent weight in lead, and they are far more efficient in allowing you to extract that energy from them. Also their form factor is much smaller making them (I hope) easier to fit into a small car like the Miata. Lastly, while you can change/discharge the typical lead acid battery 400-600 times before you kill then (depending on how far you discharge them), Lithium Ion (specifically Lithium Iron Phosphate - LiFePO4) cells have up to 2000 or more cycles in them - making them at least as cost effective (though with a higher up front cost) as lead acid as you need to replace then less frequently.

My basic research showed that the typical cost of a 3.2V LiFePO4 cell was about $1.60 retail per Ah (Amp Hour - basically the measure of capacity of a cell - 1Ah means that the cell can discharge at 1Amp for one hour before it runs out). So for a 144V battery made up of 3.2V 160Ah cells I was looking at c. $11,500 before thinking about delivery, duties (these ALL come from China at those prices and a BMS) - ouch! However as luck would have it, I was able to hook up with a group buy of cells that was being put together by someone that reduced to price to a much more palatable $1.10/Ah - or just over $8000 for the battery itself.

That battery should give me a range of about 75 miles or so. That assumes that on average I'll use 250Wh per mile which seems typical. The total capacity of that battery is 144V (45 * 3.2V) * 160Ah, or 23KWh. The battery specs say that they can be discharged 80%, making 18.4KWh available - hence the c. 75 mile range. I might get a little more or a little less, but as my commute is only 4 miles one-way and the longest trip I typically take in the baby car is to RFK Stadium and back for a D.C. United soccer game - about 60 miles round trip, that's plenty enough for me. So I plunked down my 50% deposit with Dave and James from EV Components today for the battery which should arrive in about 12 weeks.

So I've thrown my hat into the DC pot ... now I have to figure out all the other bits and pieces I need to order.

1 comment:

  1. I wish I could have went a high voltage AC route, but there just weren't any good options that the average consumer could buy. The cost increase for the little gained on those AC setups just couldn't be justified.